The present invention concerns digital communication systems and is particularly concerned with multi-station, demand-assigned, time division multiple access systems in which a population of outstations communicate with a head end or head station via a common transmission medium. For the remainder of this specification, demand-assigned, time division multiple access systems will be referred to as DA-TDMA systems.
In such systems, digital information is communicated from the head end to the outstations in such a manner that all the outstations receive the same stream of digital information and select their traffic from it under the command of the head end. Traffic is only sent to and from an outstation when and to the extent that, there is a demand for it, e.g., in a DA-TDMA system forming part of a public switched telephone network when a call is to be sent to or from an outstation.
It is a characteristic of such systems that the outstations are at random and unplanned distances from the head end. The transmission of digital information from the head end to the outstations is known as downstream transmission. The fact that the distances between the head end and the outstations are random presents no problem to downstream transmission.
Digital information transmission in the other direction from an outstation to the head end is known as upstream communication, and this does present a problem. This is because it is necessary for upstream communications to be arranged so that they do not overlap in time and thereby interfere with one another. A well known method is to transmit a burst of information from each outstation that is carrying traffic at that time in turn, and so time the transmission of these bursts that they arrive at the head end sequentially and without overlap.
In order to achieve this, there must be knowledge within the system of the times of flight of bursts of digital information from each outstation to the head end. The process of gaining this information and putting it to use so as to avoid overlap is known as "ranging". The repetition rate and accuracy required of the ranging process depends on the nature of the system, that is, whether it is static, mobile, radio, cable T.V., optical, etc.
One application of such TDMA systems is to multi-point radio, optical and co-axial cable systems with fixed or quasi-fixed outstations. In such a network, the rate of change of range is very low, and a common technique is to "range-in" an outstation when it first wishes to communicate and subsequently to observe the relative time of reception of information bursts at the head end and, if necessary, marginally correct it by sending messages to the outstation during the subsequent period of communication by the outstation.
The present invention is concerned with carrying out the initial ranging-in of an outstation on "installation".
Two methods are already known for dealing with this problem. In the first method, a "dead time" or "guard time" is kept clear in the uplink or upstream transmission, specifically for the purpose of ranging. An outstation transmits a pulse or short pulse train, when commanded to do so by the head end, within this guard time. The time of arrival of this pulse or short pulse train at the head end is then measured by the head end, and information is sent to the outstation to transmit its uplink traffic bursts data at a specific time so as to avoid overlap with the traffic from other outstations.
In the second method, ranging information is transmitted from outstations on the uplink at a low level at the same time as the wanted uplink data, using a long digital sequence with a high autocorrelation function. This ranging information signal is kept low enough in level so as not to interfere with the normal uplink data reception. At the head end, a separate part of a receiver is employed, which determines the range using a correlator to identify the most likely time of arrival of this low level digital sequence.
Both these known systems have disadvantages, particularly in the case of multi-point optical, radio and co-axial cable TDMA systems. The first method results in a quantity of unavailable transmission time and can also involve digital storage and delay of the uplink traffic. The second method is complex and involves considerable digital processing. It also normally requires multiple transmissions of the ranging signal.
The present invention is divided from United Kingdom patent application Ser. No. 9026932.5. A further United Kingdom patent application Ser. No. 9126350.9, now United Kingdom Patent No. 2252882A, has been filed, claiming priority from that first-mentioned application.